This invention relates to an apparatus and method for washing the perforated zone of a well casing in an oil or gas well. More particularly, the invention is directed to a wash tool inserted within a perforated casing at a desired depth and the method of injecting a washing fluid through the tool into the zone adjacent the perforated casing.
Heretofore, such as shown in U.S. Pat. No. RE. 31,842, dated Mar. 5, 1985, well washing tools have been provided for insertion within perforated casing for injecting washing fluid through the tool into the zone adjacent the perforated casing. Such a well washing tool or so-called straddle packer is lowered to a desired depth in the well and fluid pressure is applied through the pipe string for expanding upper and lower packers into sealing engagement with the casing at a first predetermined pressure level while valve means block fluid flow into the formation or strata located between the packers. Then, upon an increase in the fluid pressure from the pipe string to a predetermined higher fluid pressure, valve means are actuated by such higher fluid pressure to permit fluid flow into the adjacent formation through the perforated casing between the packers. Upon a relief of fluid pressure within the pipe string, the valve means are again actuated to block fluid flow into the adjacent formation.
The well washing tool as shown in U.S. Pat. No. RE. 31,842 is relatively complicated for effecting the so-called two-stage operation in which the packers are first moved into sealing engagement with the perforated casing at a first predetermined fluid pressure while fluid to the formation is blocked by suitable valve means, and then at a second higher predetermined fluid pressure the valve means is actuated to permit fluid flow to the formation through the perforated casing. An annular chamber formed between the mandrel and an outer tubular member about the mandrel receives a pair of end pistons which are urged against the packers at the first fluid pressure level and a third intermediate piston which is urged at a second higher predetermined fluid pressure to a position unblocking ports to permit fluid flow to the formation.
Such an arrangement utilizing pistons with associated seals is relatively costly and complex for a wash tool. Also, relatively high fluid pressures, such as around one thousand (1,000) psi or greater, are required in order to urge the solid elastomeric packers radially outward into sealing engagement with the casing by compression of the packers in addition to being limited to a relatively small annular spacing as a result of such radial compression.